1. Field of the Invention
The present invention relates to a converter circuit to be used in satellite broadcasting receivers and, more particularly, to a converter circuit used in low noise block-down converters (hereinafter, referred to as LNB). As seen in FIG. 4 showing an example of the satellite broadcasting reception systems, the present invention is directed for example to use in an outdoor Ku-band LNB.
In FIG. 4, there are shown an antenna 61 receiving a signal in a range of 11.7 GHz-12.2 GHz for example; a Kuband LNB 62 for converting the received signal to a range of 950 MHz-1450 MHz for example; an indoor receiver 64 to be connected to the LNB 62 by a coaxial cable 63; and a television 65 to be connected to the indoor receiver 64. The indoor receiver 64 comprises a DBS tuner 66, an FM demodulator 67, a video and audio circuit 68, and an RF modulator 69.
FIG. 5 shows a circuit block diagram of this LNB. In FIG. 5, there are shown first and second LNAs (low-noise amplifiers) 71 and 81 respectively having horizontal and vertical signals in a range of 11.7 GHz-12.2 GHz input thereto for example; first and second BPFs (band-pass filters) 72 and 82; first and second HEMT (high electron mobility transistor) mixers 73 and 83; a pair of first amplifiers 74; an L.O. (local oscillator) 76 outputting a local oscillator signal of 10.75 GHz for example; power supply 77; a pair of temperature compensating circuits 78; and a pair of second amplifiers 79.
FIG. 6 shows an example of the constructional view of this LNB. The present invention is to be embodied for the part of LNAs 71 and 81, BPFs 72 and 82, MIXs (mixers) 73, 83, and L.O. 76 of the block diagram of FIG. 5, and applied principally to the circuit board of FIG. 6. In FIG. 6, there are shown a chassis 91 for the LNB unit; an input waveguide 92; the circuit board 93 on which the circuit of FIG. 5 is mounted; a shield plate 94; the local oscillator 95, 76 of FIG. 5; and an output terminal 96.
2. Description of the Prior Art
Conventionally, there has been developed a converter circuit as shown in FIG. 2. This converter circuit is widely used for LNBs designed for reception of the European ASTRA satellite and the U.S. Ku-band satellite. For reception of the ASTRA satellite, the converter circuit first identifies and low-noise amplifies a horizontally polarized wave signal and a vertically polarized wave signal with their input frequency ranging from 10.7 GHz to 11.8 GHz (enhance specification), and then converts them into IF (intermediate frequency) signals (950 MHz to 2050 MHz). In more detail, a horizontally polarized wave signal from an input terminal T.sub.3 is amplified by low-noise amplifiers (hereinafter, referred to as LNA) 21a, 21b of a two-stage construction of HEMTs (high electron mobility transistors) and then converted into an IF signal by an HEMT-incorporated mixer (hereinafter, referred to as HEMT mixer) 23 through a band-pass filter 22 based on oscillator power fed from a local oscillator 26. Thereafter, an IF.sub.3 output of the HEMT mixer 23 is amplified to a signal of an appropriate level by an unshown succeeding-stage amplifier provided for amplifying IF signals (hereinafter, referred to as IF amplifier).
Meanwhile, a vertically polarized wave signal from an input terminal T.sub.4 is amplified by LNAs 31a, 31b of a HEMT two-stage construction, and then converted into an IF signal by an HEMT mixer 33 through a band-pass filter 32 based on oscillator power fed from the local oscillator 26. Thereafter, an IF.sub.4 output of the HEMT mixer 33 is amplified to a signal of an appropriate level by an unshown succeeding-stage IF amplifier. It is noted that oscillator power is fed from an output terminal 26a of the local oscillator 26 to the HEMT mixers 23, 33 via a Y-type distribution circuit 25 with the aid of coupling capacitors 24, 34, respectively.
In addition, reference numerals 28 and 38 denote bias circuits for inputting a bias voltage to the LNAs 21a, 21b and 31a, 31b, respectively. G.sub.3, G.sub.4 denote gate bias supply terminals for inputting a gate bias voltage to the HEMT mixers 23, 33, respectively. Reference numeral 27 denotes a power supply terminal for supplying power to the local oscillator 26.
In another converter circuit as shown in FIG. 3, a horizontally polarized wave signal from an input terminal T.sub.5 is amplified by LNAs 41a, 41b, 41c of an HEMT three-stage construction, and then converted into an IF signal by a diode mixer 43 through a band-pass filter 42 based on oscillator power fed from a local oscillator 46. Meanwhile, a vertically polarized wave signal from an input terminal T.sub.6 is amplified by LNAs 51a, 51b, 51c of an HEMT three-stage construction, and then converted into an IF signal by a diode mixer 53 through a band-pass filter 52 based on oscillator power fed from the local oscillator 46. It is noted that oscillator power is fed from an output terminal 46a of the local oscillator 46 to the diode mixers 43, 53 via a Y-type distribution circuit 45 with the aid of directional filters 44, 54, respectively.
In addition, reference numerals 48 and 58 denote bias circuits for inputting a bias voltage to the LNAs 41a, 41b, 41c and the LNAs 51a, 51b, 51c, respectively. G.sub.5, G.sub.6 denote gate bias supply terminals for inputting a gate bias voltage to the diode mixers 43, 53, respectively. Reference numeral 47 denotes a power supply terminal for supplying power to the local oscillator 46.
The converter circuit of FIG. 3 differs from the converter circuit of FIG. 2 in that the HEMTs are provided in not two-stage but three-stage construction, that the mixers used are not HEMT mixers but diode mixers 43, 53 using diodes 49, 59, and that not coupling capacitors 24, 34 but directional filters 44, 54 incorporating a ring filter are used to supply the power fed from the local oscillator 46 to the diode mixers 43, 53.
The converter circuit of FIG. 2 incorporates coupling capacitors 24, 34 that supply oscillator power to the HEMT mixer 23 for horizontally polarized wave signals and the HEMT mixer 33 for vertically polarized wave signals, respectively. Accordingly, isolation between an output of the HEMT mixer 23 and an output of the HEMT mixer 33 is dependent on these coupling capacitors 24, 34. That is, the isolation of IF signals of the 1 to 2 GHz band is dependent on constants of the coupling capacitors 24, 34. However, due to a limitation in blocking IF signals by decreasing the constants of the coupling capacitors 24, 34, the isolation characteristic for IF signals is limited, to a disadvantage.
Also, the isolation of 10 to 13 GHz RF signals, which are a horizontally polarized wave signal and a vertically polarized wave signal, is dependent on the isolation of inputs and outputs of the HEMTs used in the HEMT mixers 23, 33. It is noted that the coupling capacitors 24, 34 are in actual cases ineffective for the isolation of RF (radio frequency) signals because of too high frequencies of the RF signals. Accordingly, a problem is the mass-produced HEMTs would have variations in their input-output isolation characteristic for RF signals, which in turn would cause variations in the isolation characteristic of horizontally and vertically polarized wave signals.
The degree of cross polarization isolation (which refers to, for example, the amount of reception of a vertically polarized wave signal on the basis of the degree of reception of a horizontally polarized wave signal as viewed at the horizontally polarized wave output terminal in reception of a horizontally polarized wave signal) as an LNB using the converter circuit of FIG. 2 is at most 25 dB. In mass production, taking into consideration variations of the degree, the performance as a product would be naturally estimated to be 20 to 23 dB. Thus, a further problem is that the characteristic of 27 to 30 dB required for systems that perform digital signal processing could not be met.
The converter circuit of FIG. 3 uses diode mixers 43, 53 having no conversion gain. Therefore, in order to obtain the total gain and noise factor performance of the LNB, one has no choice but to increase the number of HEMTs that constitute the LNAs from two to three stages. This would result in an increase in the number of HEMTs, which are expensive semiconductor devices, which leads to another problem of increased cost.
The increase in the number of HEMTs would cause the printed circuit board for mounting components such as the HEMTs thereon to be also increased in size. Since the printed circuit board is also expensive, the increase in size would be an obstacle to the downsizing of the converter circuit as well as to cost reduction, resulting in still further disadvantages.